Most commercial water contains alkaline earth metal cations, such as calcium, magnesium, etc., and anions such as carbonate, sulfate, and phosphate. When combinations of these anions and cations are present in concentrations which exceed the solubility of their reaction products, precipitates form until their product solubility concentrations are no longer exceeded. These precipitates are alkaline earth metal scales. For example, when the concentrations of calcium ion and the above mentioned anions exceed the solubility of the calcium-anion reaction products, a solid phase of calcium scales will form as a precipitate.
Solubility product concentrations are exceeded for various reasons, such as evaporation of the water phase, change in pH, pressure, or temperature, and the introduction of additional ions which can form insoluble compounds with the ions already present in the solution. As these reaction products precipitate on heat transfer surfaces in contact with aqueous streams, they form scale. The scale prevents effective heat transfer, interferes with fluid flow, facilitates corrosive processes, and harbors bacteria. Scale is an expensive problem in many industrial water systems, causing delays and shut downs for cleaning and removal. Alkaline earth metal scales commonly form on the metallic surfaces of apparatuses used for thermal treatment of aqueous solutions and suspensions. By alkaline earth metal scales, we mean scales including but not limited to calcium carbonate, magnesium carbonate, calcium phosphate, and calcium sulfate. These scales form frequently in the tubes in the heat exchangers and on other heat exchange surfaces.
In the past, alkaline earth metal scale inhibition has been facilitated by the use of anionic polyelectrolytes such as polyacrylates, polymaleic anhydrides, copolymers of acrylates and sulfonates, and polymers of sulfonated styrenes. However, when used as threshold alkaline earth metal scale inhibitors, large dosages of these polymers are required. These high dosages in turn increase operating costs.
Accordingly, the need exists for an inexpensive, efficient method and composition for inhibiting the formation of alkaline earth metal scales, especially calcium and magnesium scales, on metallic surfaces in contact with water. Scale-forming compounds can be prevented by inactivating either cations or anions by chemical means with chelating or sequestering agents, so that the solubility of the reaction products is not exceeded. Polyelectrolytes, in combination with anionic surfactants, synergistically control alkaline earth metal scale precipitation, especially calcium carbonate precipitation.
It is therefore an object of this invention to provide a composition and method for the inhibition of alkaline earth metal scales over wide pH ranges. These and other objects of the instant invention are accomplished by adding an effective amount of (a) a water soluble anionic polyelectrolyte and (b) an anionic surfactant to an aqueous system, thereby preventing the deposition of alkaline earth metal scales, such as calcium and magnesium scales, on metallic surfaces in contact with said aqueous system, over a wide pH range. This process is especially effective with respect to calcium carbonate scale.
U.S. Pat. Nos. 4,427,568 and 4,324,664 disclose the use of acrylic acid/alkylhydroxy acrylate copolymers with water soluble esters of aliphatic sulphodicarboxylic acid to control scales.